OECS Regional Engineering Workshop September 29 October 3, 2014

Transcription

1 B E A C H E S. M A R I N A S. D E S I G N. C O N S T R U C T I O N. OECS Regional Engineering Workshop September 29 October 3, 2014 Coastal Erosion and Sea Defense: Introduction to Coastal Dynamics David A Y Smith

2 OBJECTIVE To provide a basic understanding of the primary coastal processes: What are the wave and current processes that dominate the nearshore zone and how do they interact with the shoreline? TOPICS Understanding Waves Waves in motion Diffraction Refraction Shoaling/Breaking Reflection Tides and Tidal Currents How are tides generated Tidal currents Sediment Transport Alongshore transport Cross-shore sediment transport

3 TYPES OF WAVES

4 WAVES IN MOTION WIND WAVE PROPERTIES Waves generated by winds transmit energy through the water column. In deep water, an orbital current flow is created below the crest of waves which reverses direction under the trough In shallow water, the orbital motion becomes a flat ellipse which eventually becomes a to-and-fro current Waves under the influence of winds are called wind waves while those that outlast this influence are called swells

5 WAVES IN MOTION WAVE PROPERTIES

6 REFRACTION WAVES IN MOTION - REFRACTION Bending of wave crests as one part of the wave crest reaches shallow water first, slowing down while the other part catches up. The distance between wave crests (wave length) decreases but the time between the wave crests (wave period) does not change. As waves approach shallow water: C = L T decreases decreases remains constant

7 WAVES IN MOTION - REFRACTION Wave orthogonal imaginary line drawn perpendicular to the crest of the wave. This essentially shows the direction of progression of the waves. Waves react to the nearshore contours and bend until they become almost perpendicular to the contours This causes focusing on some sections of the shoreline such as headlands and dispersion in other areas such as bays

8 WAVES IN MOTION SHOALING AND BREAKING Increase or decrease in wave height due to the change in depths as the wave approaches shallow water As the waves bunch closer together in shallow water, they increase in height Waves increase in height until they become so steep they break As the waves break, the Kinetic energy (from the forward motion of the waves) is converted to Potential Energy (increase in height of water) during breaking Wave crest Breaking MSL Seabed

9 WAVES IN MOTION - DIFFRACTION Diffraction occurs when waves pass through a gap or against the edge of a physical structure

10 WAVES IN MOTION - REFLECTION Vertical surfaces absorb, transmit and reflect wave energy. Reflection occurs when the wave energy that impacts on to a physical structure is not totally absorbed by the structure or transmitted through the structure. Reefs and rock breakwaters absorb and transmit some of the energy to their lee side (in the incident direction of the waves) but they also reflect a portion of this energy. Vertical walls reflect most of the energy in the opposite direction of the incident waves. Reflection can reduce or increase total wave energy depending on where in the wave cycle the impact occurs

11 THE RIVER OF SAND WAVES TO SEDIMENT TRANSPORT Waves are generated in deep water by winds. As waves approach the shoreline they undergo several transformation processes such as refraction, shoaling and breaking. As waves break, the dissipated energy causes an increase in water level Different water levels along a shoreline cause an alongshore current. The breaking process also disturbs the sediments on the seafloor, bringing them into suspension. The alongshore currents move the sediments along the shoreline.

12 SEDIMENT TRANSPORT ALONGSHORE MOVEMENT When waves break, their Kinetic Energy is transferred to Potential Energy in the form of an increased water level. The variations in water level along the shoreline cause water flow (currents) from areas of high to low water level. The turbulence from the breaking waves brings sediments on the seabed into suspension. The wave-generated currents transport these sediments along the shoreline. CROSS-SHORE MOVEMENT Waves breaking on a shoreline over a period of time shape the crossshore profile into a stable equilibrium profile. Higher waves result in steeper beach slope formations. Therefore, the profile of beaches tend to vary with the seasons. Larger than normal waves (eg. Hurricanes) can have a profound impact on a beach profile, moving sand from the shoreline to the offshore area and eroding beach dunes.

13 SEDIMENT TRANSPORT -ALONGSHORE TRANSPORT ALONGSHORE ZONE OF TRANSPORT Wave crests Depth contours S.T. Point of breaking Surf Zone shoreline

14 SEDIMENT TRANSPORT -CROSS-SHORE CROSS-SHORE - SEASONAL BEACH PROFILE CHANGES Storm or Winter Swell Waves Dune Summer waves Area of accretion Cross-shore sand movement Area of erosion Steeper beach profile after storm or during seasons of high waves

15 SEDIMENT TRANSPORT -SOURCES AND SINKS SEDIMENT SOURCES Rivers Coral Reefs Seagrass/algae Erosion of cliffs SEDIMENT SINKS Canyons Shelves Beaches Structures

16 SEDIMENT TRANSPORT IMPACTS OF STRUCTURES IMPACT OF NATURAL AND ARTIFICIAL STRUCTURES IN THE SEA ON SEDIMENT MOVEMENT AND BEACH FORMATION. Reefs Natural offshore formations that create calm areas in their lee thereby reducing the potential for cross-shore movement of sand (and alongshore movement to a lesser extent), often resulting in build up of sand on the beach. Breakwaters artificial shore-parallel reef structures that create build up of sand depending on their level of submergence and distance from the shoreline. Groynes Shore-perpendicular structures that interrupt the alongshore movement of sand. Groynes tend to create build up on the upstream end and erosion on the downstream end of the dominant sediment transport direction.

17 GROYNE IMPACT

18 BREAKWATER IMPACT Breakwaters

19 TIDES Three principal forces are involved in the production of tides: (1) gravitational attraction between the moon and the earth; (2) gravitational attraction between the sun and the earth; and (3) the force of the earth's gravity, which pulls every particle of the earth toward the earth's center. The moon is mainly responsible for the tides (its effect is about 2.2 times as great as the sun's).

20 TIDAL CYCLES Tidal currents and wave-driven currents interact to produce resulting nearshore current flows In aggressive wave environments wave-driven currents usually dominate in the nearshore Tidal currents and oceanic currents always dominate in the offshore

21 OCEANIC CURRENTS Tidal currents and oceanic currents always dominate in the offshore